The invention relates to the fuel management in internal combustion engines. More particularly, the invention relates to a method and an apparatus for use with a fuel preparation system in which an oxygen sensor in the exhaust system generates signals related to the exhaust gas composition. The fuel preparation system may be a carburetor, a fuel injection system of any type or some other fuel mixture preparation system. The method and apparatus according to the present invention provide continuous monitoring of the operational readiness of the exhaust gas sensor and initiate a switchover from closed-loop to open-loop control and back depending on the operational readiness of the sensor.
This exhaust gas sensor, usually an oxygen sensor, and sometimes referred to as a .lambda.-sensor, is used to provide the actual variable in a preferably closed-loop control system in which it is placed in the exhaust system and generates an electrical signal which is then used for a comparison with a reference value in order to provide closed-loop, i.e. feedback control, of the fuel mixture. The closed-loop control permits a very precise mixture preparation which is desirable to attain the most efficient use of fuel and an exhaust gas as free from toxic or noxious components as possible. The mixture preparation system normally contains a provision permitting operation in open-loop mode, i.e. when no control signal from the sensor can be discerned or if the signal is not adequate for use as a control signal. In such a case, the fuel mixture generator operates on a simple forward control, i.e. it meters out fuel on the basis of the prevailing position of control levers or mechanisms to as to prepare an average value of the fuel mixture ratio in overall dependence on the engine operation, for example load and engine speed. The closed-loop control is possible only for a properly functioning .lambda.-sensor. The .lambda.-sensor may fail to give a usable signal for a variety of reasons; in particular it will not be able to generate a usable output signal if its temperature is too low. Similarly, a signal may fail to be obtained if the sensor cable is broken or short-circuited. The signal from the sensor may also be erroneous and unusable for closed-loop control due to aging, a fracture of the ceramic envelope or chemical poisoning. In order to prevent engine malfunction when one of these sensor conditions obtains, it is necessary to install monitoring systems which respond to any of the aforementioned sensor malfunctions and which initiate a switchover to direct forward control until such time as the .lambda.-sensor again becomes operational.